1. Field of the Invention
The present invention relates to a method for automatically developing a geological model from seismic data. It finds particularly relevant applications in the field of the analysis of seismic data, namely during the search for new oil fields.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Generally, the seismic data represent a subsoil and therefore permit to apprehend the geological structure of the covered area. A seismic signal typically represents the derivate of the acoustic impedance of the layers through which pass sound pulses emitted during the acquisition of said seismic data. Therefore, the seismic line is an indirect and degraded representation of the underlying geology. Indirect because the value of the pixel is not representative of the very nature of the subsoil at that point, and degraded because the final data are the result of a chain of processing operations (deconvolution, <<stacking process>>), which introduce artifacts and approximations. Therefore, the reconstruction of the geology from the seismic data generally requires complex analysis means, which try to identify the geological layers one by one and independently from each other. The existing analysis methods use as a postulate that one and the same geological layer has similar seismic signatures. The surfaces are thus extracted through propagators that start from a germ, or by means of classifiers (Kmean) that group the seismic signatures having the same gauge. The main limitation of these techniques results from the basic postulate itself, which is never really maintained. In addition, the algorithms used have many drawbacks: too strong influence of the germ, drifts of the propagations, invalid surfaces connecting various geological stages, etc.
More recently global approaches appeared, which aim to calculate, during one and the same process, the whole geological model directly from the seismic data. Among these methods, two large families can be distinguished: those using the local gradient of the layers and those using primitives.
In the family of methods using the local gradient of the layers, namely U.S. Pat. No. 6,771,800 belonging to Keskes et al. (2002) is known, which is based on the propagation of 2D lines in the direction of the gradient. This technique works only in 2D and produces no interruptions of continuities of the geology.
Also known is the paper by Lomask et al., <<Flattening without picking>>, Geophysics vol. 11 no. 4, 2006, pp. 13-20, which provides a method for compensating for the gradients by means of an iterative minimization process. During this process, the gradients, obtained by calculating the 3D gradient on each seismic point, are gradually eliminated, until a flat seismic block, without global deformations, is obtained, which permits to reveal the sedimentation rates by means of a stratigraphic analysis. This technique works in 3D, but the management of the interruptions of the model requires an a priori knowledge of the discontinuities.
In the family of methods using primitives as basic elements of the geological model, these elements are, in a first step, extracted from the seismic image and are then arranged with respect to each other so as to construe the model.
In the method described by Monsen et al., namely in US 2008/0140319, the primitives are varying-size geological surfaces obtained automatically by propagation on the extrema of the seismic line. These primitives are then sorted by topological order based on an acyclic graph (<<Directed Acyclic Graph>>, DAG), which represents the superposition relationships between the primitives. The topological order assigned to each surface permits to construe the geological model. This method works in 3D, it manages the erosion-like discontinuities, but does not permit to automatically manage the faults.
A variant of this approach is provided by Verney et al. in <<An Approach of Seismic Interpretation Based on Cognitive Vision>>, 70th EAGE Conference & Exhibition—Rome, Italy, 9-12 Jun. 2008. In this document, the relationships between mini-surfaces are hierarchized depending on their mean vertical distance and the fusion criteria between primitives are based on principles from the cognitive science.
In the present context of an increasingly more competitive and expensive oil market, the search for new fields requires the use of more accurate and quicker automatic analysis tools. The object of the present invention is a method working at the scale of the seismic point for automatically developing a geological model from seismic data, which can automatically manage the geological discontinuities such as the faults, and which permits to implement multi-scale strategies.